Power efficient class D amplifiers, e.g., amplifier 10 shown in FIG. 1, convert an input waveform into a continuously pulse-width modulated (PWM) analog signal. Signals with a bandwidth well below the pulse frequency may be reproduced. The output of a Class D amplifier contains unwanted spectral components, i.e., the pulse frequency and its harmonics, that must be removed by a passive LC filter 18. The resulting filtered signal is then an amplified replica of the input.
Generally, as shown in FIG. 1, a switching stage 12 of power transistors 14, 16 entails a trade off between efficiency and Electromagnetic Interference (EMI) emitted as a by-product of its normal operation. EMI causes unwanted signals (interference or noise) to be induced in the circuits. Higher dV/dt is required for the switching stage 12 to gain better efficiency. However, because of its steep transient artifacts, higher voltage change over time (dV/dt) can be a cause of EMI noise.
It is in the nature of an audio signal in a class D audio application, that the average output power is significantly smaller than its peak value. If an amplifier has a low EMI noise emission at a lower output power level and higher efficiency at a higher output power level then these two contrary design issues can be compatible.